Creating a custom ValidationAttribute class that returns multiple error messages

Question

I have a model like this:

[IsValidInput]
public class Input
{
    //different properties
}

With a custom validation attribute like this:

[AttributeUsageAttribute(AttributeTargets.Class, AllowMultiple = true, Inherited = false)]
public class IsValidInput : ValidationAttribute
{
    public override bool IsValid(object value)
    {
        try
        {
            ExternalValidator.Validate(value);
        }
        catch (CustomException ex)
        {
            foreach(var errorText in ex.GetDescriptions())
            {
                this.ErrorMessage = this.ErrorMessage + errorText;
            }
            return false;
        }
        return true;
    }
}

Now I have one ErrorMessage object that contains multiple errors. I want to somehow return multiple ErrorMessage objects, so that in my view I will have a list with multiple list-items, like this:

Validation error 1
Validation error 2

How can I return a list of ErrorMessages to archieve this?

Try using Environment.Newline to separate them and then style the html element with white-space: pre-line; — user3559349
– user3559349, Commented May 5, 2015 at 13:30
I can output the messages on different lines thanks to the answers for this question: stackoverflow.com/questions/12909351/… But that's not what I'm looking for because there would be only one bullet point. (Still only one item in the HTML list). — MmM
– MmM, Commented May 5, 2015 at 13:34

Pawel Maga · Accepted Answer · 2015-05-05 13:49:52Z

3

Hi you can return simple ValidationResult class instead of boolean:

 public class ValidationResult
 {
    public bool IsValid { get; set; }
    public IList<string> Errors { get; set; }

    public ValidationResult()
    {
        Errors = new List<string>();
    }
 }

public class IsValidInput
{
    public ValidationResult IsValid(object value)
    {
        ValidationResult result = new ValidationResult();

        try
        {
            ExternalValidator.Validate(value);
            result.IsValid = true;
        }
        catch (CustomException ex)
        {
            foreach(var errorText in ex.GetDescriptions())
            {
                result.Errors.Add(this.ErrorMessage + errorText);
            }
        }
    }
    return result;
}

edited May 5, 2015 at 13:49

answered May 5, 2015 at 13:44

Pawel Maga

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4 Comments

MmM Over a year ago

Interesting, I'll try this!

Pawel Maga Over a year ago

I would recommend also to be interested with FluentValidation - fluentvalidation.codeplex.com

MmM Over a year ago

I haven't managed to get my result this way. It did point me to the class System.ComponentModel.DataAnnotations.ValidationResult but as if I understand correctly this is also limited to one error message. msdn.microsoft.com/en-us/library/…

Pawel Maga Over a year ago

You cant reach that using this attribute with this overrided method. Consider use custom validation class to validate objects

MmM · Accepted Answer · 2016-07-01 13:03:52Z

2

I've found a workaround:

I'll append my error message with some html tags like this:

foreach(var errorText in ex.GetDescriptions())
{
    this.ErrorMessage = this.ErrorMessage + txt + @"</li><li>";
}
this.ErrorMessage = this.ErrorMessage.Remove(this.ErrorMessage.Length - 4);

And add @Html.Raw in my view:

@if (Html.ValidationSummary() != null) { @Html.Raw(HttpUtility.HtmlDecode(Html.ValidationSummary().ToString())); }

This will give me the html list with validation results that I want.

edited Jul 1, 2016 at 13:03

answered May 5, 2015 at 15:09

MmM

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1 Comment

Timothy Over a year ago

But it only renders string 'Microsoft.AspNetCore.Mvc.Rendering.TagBuilder'

Carl Prothman · Accepted Answer · 2015-05-05 13:34:53Z

Check out this solution: http://www.codeproject.com/Articles/234096/Multiple-Custom-DataAnnotations-on-Same-Field-With

1) Use a static field which will keep track of how many attributes per field or property there are, and as per the count, appends letters a, b, c... in the ValidationType of each next rule produced for the field or property.

2) Provide a custom HTML Helper to render the editor for the field; the HTML Helper will then parse all "HTML-5 data-val" attributes on the field and will convert them to a requiredifmultiple rule (client side rule which doesn't change anything on the server side code) for the field.

3) Provide two adaptors and validation functions for the client side validation of this custom validation attribute, one if there is only one instance of the Attribute on the field (i.e., RequiredIf), another when there are multiple instances of the Attribute on the field (i.e., RequiredIfMultiple).

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